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Three stealthy tracking mechanisms designed to avoid weaknesses in browser cookies pose potential privacy risks to Internet users, a new research paper has concluded.

The methods -- known as canvas fingerprinting, evercookies and cookie syncing -- are in use across a range of popular websites. The findings, first reported by Pro Publica, show how such tracking is important for targeted advertising but that the privacy risks may be unknown to all but the most sophisticated web users.

Profiling Web users, such as knowing what Web pages a person has visited before, is a central component of targeted advertising, which matches advertisements with topics a person may be interested in. It is key to charging higher rates for advertisements.

Cookies, or data files stored by a browser, have long been used for tracking, but cookies can be easily blocked or deleted, which diminishes their usefulness.

The methods studied by the researchers are designed to enable more persistent tracking but raise questions over whether people are aware of how much data is being collected.

The researchers, from KU Lueven in Belgium and Princeton University, wrote in their paper that they hope the findings will lead to better defenses and increased accountability "for companies deploying exotic tracking techniques."

"The tracking mechanisms we study are advanced in that they are hard to control, hard to detect and resilient to blocking or removing," they wrote.

Although the tracking methods have been known about for some time, the researchers showed how the methods are increasingly being used on top-tier, highly trafficked websites.

One of the techniques, called canvas fingerprinting, involves using a Web browser's canvas API to draw an invisible image and extract a "fingerprint" of a person's computer.

It was thought canvas fingerprinting, first presented in a research paper in 2012, was not in use on websites. But it is now employed on more than 5,000 of the top 100,000 websites ranked by metrics company Alexa, according to the paper.

More than 95 percent of those canvas fingerprinting scripts came from AddThis.com, a company that specializes in online advertising, content and web tracking tools. AddThis.com could not immediately be reached for comment.

The researchers also found some top websites using a method called "respawning," where technologies such as Adobe System's Flash multimedia program are manipulated to replace cookies that may have been deleted.

These "evercookies" are "an extremely resilient tracking mechanism, and have been found to be used by many popular sites to circumvent deliberate user actions," the researchers wrote on a website that summarized their findings. Respawning Flash cookies were found on 107 of the top 10,000 sites.

The third method, cookie syncing, involves domains that share pseudonymous IDs associated with a user. The practice is also known as cookie matching and is a workaround for the same-origin policy, a security measure that prevents sites from directly reading each other's cookies. Such matching is helpful for targeting advertisements and for selling those ads in automated online auctions.

The researchers argue that cookie syncing "can greatly amplify privacy breaches" since companies could merge their databases containing the browsing histories of users they're monitoring. Such sharing would be hidden from public view.

Those companies are then in "position to merge their database entries corresponding to a particular user, thereby reconstructing a larger fraction of the user's browsing patterns."

"All of this argues that greater oversight over online tracking is becoming ever more necessary," they wrote.